1. Field of the Invention
This invention relates to a heat developable photosensitive material and more particularly to a heat developable photosensitive material having a laminated structure with improved contact between each layer.
2. Description of the Prior Art
A heat developable photosensitive material is a photographic material that can be dry-processed without using any liquid. As illustrated in U.S. Pat. Nos. 3,152,904 and 3,457,075, the essential components of the material are an organic silver salt such as silver behenate, a reducing agent for the organic silver salt, and a photocatalyst such as silver halide that releases, upon exposure to light, a substance which serves as a catalyst for the reaction between the organic silver salt and the reducing agent under heating. Such a photographic material is stable at ordinary temperatures but when it is heated to a temperature of at least 80.degree. C., preferably at least 100.degree. C., after imagewise exposure, the oxidation-reduction reaction between the organic silver salt (oxidizing agent) and the reducing agent in the photosensitive layer occurs as catalyzed by an exposed photocatalyst in proximity with the agents, and the resulting silver causes the exposed area of the photosensitive layer to rapidly darken to provide an image having contrast with the unexposed area (background).
The simplest construction of a heat developable photosensitive material comprises a support having coated thereon a single layer containing all the stated essential components. But in most practical applications, each component is incorporated in a separate layer to prevent it from entering into dark reaction with other essential components or other additives (e.g., toner). Alternatively, as taught in Japanese Patent Application (OPI) No. 87721/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a subbing layer is disposed between the support and the single layer to prevent heat fogging, or as described in Japanese Patent Application (OPI) No. 6917/74 (U.S. Pat. No. 3,933,508), a polymer overcoat is disposed to increase the transparency of the film or improve its keeping quality with time.
However, it was found that due to poor contact between each layer of the laminated structure, two adjacent layers easily peeled from each other, resulting in a defect called "delamination". The phenomenon of poor adhesion of laminated layers has been studied for many years in the art of common gelatin-silver halide photographic sensitive materials, but in the field of heat developable photosensitive materials, this is a new problem and few prior art references suggest a solution to it.
In some cases, delamination in a heat developable photosensitive material occurs during heat development due to a heat developing machine using a dirty developing rollers and, in other cases, it occurs when a pressure sensitive adhesive tape used to attach a heat developed photosensitive material to the wall is peeled from the wall. The defect is developed in the interface of any two layers of the heat developable photosensitive material, but the interface between the polymer overcoat and a layer beneath it is most susceptible to such defect. Delamination results in a broken copy image or the least legible image.